1. Field of the Invention
The invention is directed to a power amplifier of the type suitable for feeding an inductive load having switched transistors.
2. Description of the Prior Art
U.S. Pat. No. 5,113,145 discloses such a power amplifier wherein one bridge half is provided for each direction of current through the inductive load. A first bridge arm having a parallel group of series circuits of transistors with free running diodes and a second bridge arm having a parallel group of series circuits of free running diodes with transistors are respectively arranged between supply voltage terminals. The inductive load is connected between the junction points of transistors and free running diodes of the first bridge arm and the junction points of transistors and free running diodes of the second bridge arm.
Power MOSFETs are thereby employed, which enable higher switching speeds and, thus, reduced switching losses compared to bipolar transistors. By contrast to bipolar transistors, moreover, MOSFETs do not have a turn-off delay retarded by storing time, so that the regulatability of the power amplifier is improved. One disadvantage of power MOSFETs compared to bipolar transistors is the low current-carrying capability of MOSFETs. Many MOSFETs must be connected parallel when high currents are required of a power amplifier.
High demands are made on power amplifiers, for example, in the case of gradient amplifiers of nuclear magnetic resonance tomography systems. Coil arrangements for generating linear magnetic field gradients must be supplied with current via such power amplifiers. For example, the following, typical demands are made on such power amplifiers:
a) The currents must be capable of being exactly set within a broad range; PA0 b) Currents in two directions are required; PA0 c) The curve shape of a current prescribed by a drive must be reproduced as precisely as possible; PA0 d) The power amplifier must deliver an output voltage that assures an adequate rise rate of the current of the gradient coil; PA0 e) The power amplifier must allow an optimally high "duty cycle" given an optimally high nominal current; and PA0 f) The power amplifier must be as compact as possible.
With the parallel connection of many transistors and a desire to have a high switching speed, the problem of uniform distribution of current among the parallel transistors arises. Particularly when switching events occur, there is the risk that the distribution of the current will be non-uniform, for example, due to line inductances, potentially leading to the destruction of transistors.
According to the initially cited U.S. Pat. No. 5,113,145, this problem is resolved by securing the parallel transistors on a thermally and electrically conductive, first ring, with the transistors uniformly distributed around this ring and respectively electrically connected thereto with one terminal. The connections to further terminals of the transistors ensues substantially rotational-symmetrically via large-area printed circuit boards.
Due to the symmetrical arrangement of the transistors in circular form and due to a planar execution of the connecting lines, uniform distribution of the current to be switched among the individual transistors is assured at any point at time, even given the highest switching speeds that can be achieved. An inductor is provided in each bridge arm in order to prevent formation of the parasite diode, which is inherent in every MOSFET transistor and which exhibits a relatively long reverse recovery time, from causing an inadmissibly high quadrature-axis component of the current. The load current can thus only commute to an inherent diode via these inductors. In order to keep the structural volume of the inductors within limits, a saturation of the inductor cores by the output current is avoided by means of a current compensation of the inductors.
An extremely compact structure is achieved in this known arrangement. Due to the rotational-symmetrical arrangement, however, specific cooling members that are relatively complicated in terms of manufacture and printed circuit boards are required. This known power amplifier is designed for currents up to 250 A and output voltages up to .+-.300 V. Given employment of fast pulse sequences in nuclear magnetic resonance tomography, however, these values are not adequate. Given higher dissipative energies of the semiconductor components, the dissipated heat can not be economically eliminated by means of air cooling. At higher operating voltages, the known structure is also problematical because in the possibility of creep paths arising.